The invention relates to a brushless direct-current motor, in particular to a commutation circuit for such a motor. The system involves motors with a permanent magnetic rotor with an arbitrary number of pole pairs and a stator with at least two windings, which form a multiphase system. Each phase can be switched by means of electronic switch elements as a function of voltages that are induced in the windings by the permanent magnetic field of the rotor. Depending on the state of commutation, this leads to the performing of commutation steps at the negative and/or positive poles of a direct-current source.
If direct-current motors are commutated electronically, then the instantaneous rotor position must be ascertained each time. For this purpose, sensors are typically used, such as Hall generators or photodiodes.
It is also known for the voltages induced in the armature windings to be measured at whichever instants are currentless, and thus to detect the rotor position and from that to obtain appropriate signals for stepped switching of the commutation.
German Patent Application DE 26 04 638 A describes a control circuit for a collectorless direct-current motor. There, the algebraic sign pattern of the voltages induced in the three phases in relation to a reference voltage is utilized for commutation. To do so, however, at any arbitrary instant, at least one of the phases has to be currentless. This means that for a certain period of time, such as one complete half-wave, the current must be switched off so that the induced voltage can be determined with the requisite safety. Because of the common reference potential necessary for measuring the level, this control circuit can be used only for star-connected motors with a star center point extended to the outside. Moreover, that circuit furthermore requires a special starting logic.
It is accordingly an object of the invention to provide a system for sensorless commutation, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for a commutation circuit for a brushless direct-current motor that is simple in design and economical to produce, that requires no angle detecting device built into the machine or mounted on it, and that reliably step-switches the stator rotary field from the angular position of the rotor in any rpm range.
With the foregoing and other objects in view there is provided, in accordance with the invention, a commutation circuit for a brushless direct-current motor having a motor star circuit. The novel circuit comprises:
a virtual star circuit analogous to the motor star circuit and formed of a plurality of resistors, the virtual star circuit and the motor star circuit each having a star center point carrying a voltage;
a comparator device connected to the virtual star circuit and to the motor star circuit for continuously comparing the voltages of the star center points of the motor star and of the virtual star in all phases and for outputting a comparison value; and
a device connected to receive the comparison value and for generating a commutation signal from the comparison value for controlling a commutation of the direct-current motor.
In other words, analogously to the motor star, a further, virtual star is formed of resistors. The voltages present at the star center points of the two stars are compared with one another and evaluated. The zero-point crossovers are evaluated for the correct-angle commutation of the various phases.
For startup, a ramp-function generator is used, for instance in the form of a pulse generator. When a certain torque is reached, this ramp-function generator is no longer needed.
The signals obtained from the voltage comparison are supplied to the counter and decoder circuit and from there are carried on to the transistors.
It is also possible to ignore the fact that there is no linear relationship between the currents and voltages of the real machine on the one hand and the virtual star circuit, formed of resistors, on the other, because compensation takes place between each two or three variables X, Y and Z.
The invention attains the stated object in that by evaluation of the comparative measurement between the motor star center point and the virtual star center point, the electromotive force (EMF) of the motor is detected continuously, and thus the values of all three phases are measured continuously.
The effort and expense for evaluating the EMF is relatively slight. The instant of commutation is determined very precisely, and in any case more precisely than with a mechanical position generator. The circuit is economical and is not vulnerable to malfunctions.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in generatorless commutation, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.